Birds of Paradise Change Eye From Blue to Yellow

Dive into the colorful world of the birds–of–paradise. Learn about how color works, and the ways birds–of–paradise exploit the properties of light to develop bright colors on their feathers, skin, and even inside their mouths.

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Pigment

Many bird-of-paradise feathers get their colors from pigments. Almost all yellow, orange, red, brown, and black colors in birds are due to pigments. Pigments are chemicals that interact with light on a molecular level, absorbing white light and emitting only certain wavelengths. We perceive the emitted wavelengths as the color of the feather. While this may sound exotic, it really isn't. Pigments are everywhere—paints, vegetables, even our own skin. The makeup of the pigment molecules determines the color emitted. Most of the red, orange, and yellow found in birds are due to chemicals called carotenoids. Most browns and blacks come from melanins, which is the same group that colors mammalian skin and hair.

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  Pigment molecules are deposited in an irregular pattern of granules on the walls of the translucent keratin cortex that make up the tiny feather barbs.

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  The yellow and red on the back of Wilson's Bird-of-Paradise are particularly vibrant examples of likely carotenoid pigments. The brown wing feathers and black face are from melanin pigments. Surprisingly, the blues are not pigments at all, but we'll deal with those separately.

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  Goldie's Bird-of-Paradise and other members of the genus Paradisaea have a range of carotenoid yellows and oranges alongside browns and blacks.

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  The underside of the King-of-Saxony Bird-of-Paradise (seen here from below), appears almost stained with yellow pigment. The head and upper back are black.

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  The yellow pigment of the Twelve-wired Bird-of-Paradise is particularly bright. There's something different about this yellow than is found in any other bird-of-paradise.

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  The yellow pigment of the Twelve-wired Bird-of-Paradise turns white after death and also in captivity without a special diet. That's not true for any other bird-of-paradise. Why? We don't know.

Structural Colors

There are no known blue pigments in bird feathers or skin. Blue pigments are rare in nature because they require metals that are hard to access and can be toxic; but that hasn't stopped birds-of-paradise from creating stunning blue ornaments. These blues are called structural colors because they are created by the way light interacts with surfaces and spaces on a microscopic scale.

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  Blue in feathers is formed by a precise arrangement of keratin that scatters blue wavelengths and lets the rest pass through. An underlying layer of dark melanin pigment removes other colors from the reflected light. What we see are feathers that are blue from many angles, almost as if they were pigmented.

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  The Blue Bird-of-Paradise is possibly the only one of the 39 species that has blue feathers with this kind of structural color. Jays and bluebirds of North America are more familiar examples of this color mechanism.

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  One of the most unusual ornaments in these birds is the blue skin on the head of the Wilson's Bird-of-Paradise. This is structural as well, but it is likely based on two-dimensional collagen arrays, a different structure with the same effect.

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  The same mechanism is likely responsible for blue legs like those of the Magnificent Bird-of-Paradise.

Note: One of the rare examples of a natural blue pigment is the blue in the shells of some bird eggs. This pigment has never been found in skin or feathers.

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Iridescence

Sometimes the color in feathers has a now-you-see-it, now-you-don't quality called iridescence. Displays that use iridescent colors are directional; they only look right from specific angles. When the feather turns a small amount, the color changes hue. When the feather turns a little more the bright color can completely disappear, revealing black or brown pigments within the light-bending layers. Most of the green, blue, and violet feathers in the birds-of-paradise are iridescent.

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  Iridescence is a kind of structural color based on the precise arrangement of keratin, melanin, and air. This illustration shows the simplest arrangement. At each layer boundary, different wavelengths bend more than others. Because of the precise geometry, only certain wavelengths are bent all the way back out. The rest are absorbed or canceled out. Depending on the angle of view, the same feather is seen as cyan, blue, or violet.

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  The iridescent effect can be seen best in a series of images. In the Magnificent Riflebird, the hue shifts as this male twists his head. When he turns away a bit, the effect disappears and all that remains is black.

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  The Magnificent Bird-of-Paradise has a deep green breast shield. For this display, the light angle seems to brighten the color more than change its hue. When the same feathers are seen from the side, they appear brown.

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  The brilliant cyan feathers of the Superb Bird-of-Paradise are jet black when seen from the side.

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  The Splendid Astrapia has a splendid example of iridescent orange on its chest and in a line on its face. You can tell that this is a structural color, not a pigment, since the color disappears when seen from the side.

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  Very few images of the Twelve-wired Bird-of-Paradise are at the proper angle to catch two remarkable iridescent traits, a green ropelike pattern worn across the belly and shoulders, as well as a deep violet sheen on the top surface of the wings. It is very likely that, from where an interested female stands, these traits are key components of the display. The precision of the alignment determines the saturation and intensity of the color.

Something Unusual

Some of the colors in birds-of-paradise defy easy categorization. Here are a few examples:

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  The iridescent breast feathers of Lawes's Parotia were recently discovered to have barbules with a unique "boomerang" shape, which allows color to abruptly shift from yellow to blue. The discovery of multiple reflectors making very different colors within a single feather is totally new to science.

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  Images of Wahnes's Parotia show hue changing rapidly across a wide spectrum, suggesting that their breast feathers use the same mechanism.

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  It isn't always obvious which mechanism creates a color. A great example of this is the King Bird-of-Paradise, which probably owes its brilliant red to a carotenoid pigment. But the hues of some parts of the body appear to change with angle, sometimes even looking black, so perhaps a structural mechanism is also involved.

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  The face of the Long-tailed Paradigalla has skin patches of non-iridescent yellow, blue, and red. If each color is produced the usual way, this is a combination of three different mechanisms stacked into one spot. Yellow is usually a carotenoid pigment; blue skin is usually structural; and most red in skin is due to blood exposed by capillaries near the skin. But each of these colors can be made by other mechanisms and, in the birds-of-paradise, unusual is par for the course.

Colorful Females

All of the really showy birds-of-paradise are males, but in some of those species females reveal hints of that coloration. In monogamous species there is mutual mate choice so females may show colors as a signal to males that they are a good choice. But for most species of birds-of-paradise, males typically mate with any willing female. In this scenario, female colors are unlikely to be a display. They are more likely a result of less selection for camouflage. And in some cases, like in the blue head of the Wilson's Bird-of-Paradise, it may simply be hard to uncouple the genes for the blue bald head and feet from sex linked expression, whereas plumage seems to be easier to keep separated by gender lines.

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  Wilson's Birds-of-Paradise have the most unusual females. They are camouflaged everywhere they have feathers, but they have the male's blue head and legs. This hints at an underlying difference in basis for color in plumage and skin.

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  The Magnificent Bird-of-Paradise is the Wilson's closest relative. While it lacks the bald blue head, its females show the same pattern of camouflaged feathers and colorful other parts, in this case blue legs and bill.

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  The Blue Bird-of-Paradise is a good example of when females look more or less like drab versions of the males. They lack the brightness and plumes but have the same overall effect.

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  Ribbon-tailed Astrapia females don't have the intensely iridescent colors of the males, but they do have (structurally) colored feathers of the head and neck.

Black & White

Black and white are more than just the absence or presence of full-spectrum light. Each is specific, and the two are often paired to dramatic effect. Click through the gallery to see some examples of this paring.

Black is a result of melanin pigment, but some blacks are more pure and flat than others. It is not clear how many variations of black pigment there are in the birds-of-paradise, but the differences in blacks, from dull to deep, are probably determined more by the microscopic anatomy of the feather surface than differences in black pigment. This is analogous to the way normal black fabric and black velvet may use the same dye but to noticeably different effects.

There is no white pigment in birds. Bright white is structural, but in a different way than other colors. The process is called incoherent scattering and it requires randomness in the keratin along with an absence of pigment. In these circumstances all colors of light are scattered equally and the effect is white. But just as with blacks, there are dirty whites and bright whites. The brightness of the white may be due to the complete elimination of all traces of pigment, or it may be due to a more effective scattering array.

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  Ribbon-tailed Astrapia

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  Splendid Astrapia

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  Bronze Parotia

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  Western Parotia

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  Blue Bird-of-Paradise

Vision

After all of this, it is reasonable to ask whether birds see these colors the same way we do. The answer is that they see all we see and more. While no one has specifically examined the vision of a bird-of-paradise, birds have better color vision than people in two ways.

1. Most birds have four kinds of color receptors (called cone cells) in their eyes. We have three. The extra kind allows them to see into the ultraviolet spectrum, where they can sense entire visual signals that are imperceptible to humans. It is entirely possible that there are color ornaments that are critical to the birds, but totally invisible to us.
2. Birds also can discern smaller differences between colors than people can. That's because they have oil-droplet filters that sharpen the sensitivity of each kind of cone cell.

So as bright as their world looks to us, it might look even brighter to them!

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Source: https://www.birdsofparadiseproject.org/color/gallery/?__hstc=75100365.3ecd7c9674ba06f08984886db67907ba.1646611200210.1646611200211.1646611200212.1&__hssc=75100365.1.1646611200213&__hsfp=952507618

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